The reduction of sulfur content in gasoline and diesel oil is an important means for improving air quality because sulfur in transportation fuel can irreversibly poison noble metal catalysts found in automobile catalytic converters. Further, due to their high energy density, ease of storage, and well-established distribution-infrastructure, transportation fuels such as gasoline, jet fuel, and diesel oil are perfect candidates for high efficiency fuel cells. Nevertheless, to protect the reforming catalyst and the electrodes of the fuel cell system from deactivation, the sulfur concentration of the fuel needs to be ultra-low (e.g., less than 0.1 ppm). Thus, there has been a growing demand for ultra-low sulfur fuels driven by legislative pressure and, in particular, the growing application of fuel cells. To deliver these ultra-low sulfur fuels, ultra-deep desulfurization of gasoline and diesel oil has become an increased focus of research with many diverse approaches.
One approach is hydrodesulfurization (HDS). Conventional HDS utilizes alumina- and silica-supported cobalt or nickel or molybdenum catalysts. However, despite significant improvements in decreasing sulfur content, it is still difficult to achieve essentially sulfur free fuels with the typical catalysts used for HDS. This is commonly believed to be due to the slow reactivities of sterically hindered dimethyl dibenzothiophenes [1-5] and other thiophene-based compounds, examples of which are shown in FIG. 1. Typical catalysts for hydrogenation and conventional HDS are transition metals of the Group-9 and Group-10 elements, such as cobalt, nickel, rhodium, palladium, and platinum. Nickel (Ni) and cobalt (Co) with various promoters and on various supports are extensively used, and carbon-sulfur cleavage under hydrogen pressure is often achieved via a classical hydrogenolysis reaction scheme. Indeed, extensive studies on the catalytic properties of transition metal catalysts have shown that Ni and Co have significant catalytic activity [9-14], with conventional catalysts like Co(Ni)—Mo/Al2O3 achieving bulk sulfur removal from gasoline and natural gas with residual organo-cyclic sulfur compositions down to 30 wt ppm [15]. Further, in a hydrogen atmosphere, the metal active phase, i.e. nickel, is able to cleave the sulfur off the “difficult” cyclic compounds; thereby converting the sulfur into hydrogen sulfide (H2S). However, in these catalysts, the active nickel phase is slowly sulfided by the resulting H2S forming NiS, as shown in the following reaction:Ni+H2S→NiS+H2 ΔG(500K)=−42.3 kJ/mol  (1)
As a result of the above reaction, as nickel undergoes conversion to NiS, the adsorption activity decreases and eventually vanishes completely. In industry, the catalyst will then either have to be regenerated or replaced once a certain threshold breakthrough of sulfur is detected in the product stream. Consequently, there are several actively researched alternative methods that aim at removing sulfur to levels below 10 ppm, including oxidative routes for diesel, chemical conversion methods, non-destructive adsorption, extraction, biodesulfurization, and, in particular, reactive adsorption [5-8]. In reactive adsorption, once a conventional metal active phase of a catalyst converts all “difficult” organic sulfur species under hydrogen to H2S, an adsorptive phase, usually the base oxide support material, accepts, and permanently stores the sulfur portion of the H2S, as storage which is typically necessary as H2S, when present downstream of the catalyst adsorbent, tends to recombine with olefins to form relatively stable mercaptanes.
With further regard to reactive adsorption, it is appreciated that a nickel active surface can catalyze the cleaving of cyclic sulfur compounds such as thiophenes. As indicated above, however, during the process, the active Ni sites get sulfided to NiS thereby extinguishing catalytic activity. As such, a catalyst that retains its catalytic activity over an extended period of time during desulfurization would be both highly desirable and beneficial.